Personally I think having the inlet on the side is an awesome idea. You'll notice in the body of your DC, that's how it works! Not only does it eliminate an elbow like Phil said, but it also takes that elbow OUT of the way! So when the stuff starts spinning, it just keeps going and going. I bet you get better cyclonic action (but that's just a gut feeling...). If I were to do one, I'd do it that way.
As for the upside down thing, rather than trying to use the laminate or other internal material to locate the upper half, how bout screwing a lip to the outside of one half? Either take a flexible strip or something and bend it around the drum creating a lip, or just screw a bunch of wooden tabs around the outside? Or glue them maybe? Depending on the material of the drum itself...
And to keep the bag in place, formica, screen material, whatever's cheap!
As for the upside down thing, rather than trying to use the laminate or other internal material to locate the upper half, how bout screwing a lip to the outside of one half? Either take a flexible strip or something and bend it around the drum creating a lip, or just screw a bunch of wooden tabs around the outside? Or glue them maybe? Depending on the material of the drum itself...
And to keep the bag in place, formica, screen material, whatever's cheap!